As the yield and efficiency of light emitting diode (LED) technology has improved, LED print bar (LPB) imagers have been developed and used for xerographic printing applications, in higher performance and higher quality applications. For yield reasons, optical performance and compactness, full width LPBs, i.e., LPBs spanning the entire cross process direction, are often made as multi-chip assemblies carefully assembled and focused in a housing with a SELFOC® lens array, i.e., a gradient index lens array or GRIN lens array, as shown in FIG. 1. For clarity, the housing has been omitted in FIG. 1. SELFOC® lens array 50 is arranged between multi-chip LED array assembly 52 and photoreceptor drum 54. It should be appreciated that although a photoreceptor drum is depicted in FIG. 1, other photosensitive surfaces may also be used in the foregoing arrangement, e.g., a photoreceptor belt. During xerographic printing, LED light 56 from array assembly 52 is focused on drum 54 via lens array 50. The “self-focusing” property of SELFOC® lenses is well known in the art and therefore not further described herein.
As shown in FIG. 2, SELFOC® lens 50 may be formed from a plurality of gradient index lens 58 within housing 60. Housing 60 may include angled wall 62 which causes lenses 58 to align in two rows, wherein the second row is offset from the first row. In an embodiment, the longitudinal axis of each lens 58 in the second row is the aligned with the point of contact between two adjacent lenses 58 in the first row.
Due to the construction methods and characteristics of LEDs, LED chips and lenses, a LPB has imperfect imaging characteristics which can negatively impact print quality. For example, chips 64, each comprising multiple LEDs 66, are placed on a substrate, e.g., printed circuit board 68, as accurately as possible, but due to some variability in placement there are non-idealities in chip gaps and linear placement of chips 64 on the multi-chip substrate, as depicted in FIGS. 3 and 4.
Adjacent chips may be offset in the X or Y direction relative to each other. For convenience, X and Y directions are set forth on FIGS. 3 and 4. Moreover, adjacent chips may be angularly rotated relative to each other. As the foregoing non-idealities may be additive across the length of printed circuit board 68, they can contribute to bow (bi-directional arrow 70), skew (bi-directional arrow 72) and magnification error, i.e., the sum of between chip offsets in the X direction. It should be appreciated in view of FIGS. 3 and 4 that “P” is used to represent the spacing between individual LEDs 66 within a single chip 64, “PG” is used to represent the spacing between adjacent LEDs 66 within adjacent chips 64, “DY” is used to represent the difference is the Y direction, i.e., process direction, between the average position of LEDs 66 within a first chip 64 relative to the average position of LEDs 66 within a second chip 64 adjacent to the first chip 64, and “DX” is used to represent the difference between “PG” and “P”. Thus, the absolute magnification error is equal to the sum of “DX” for all chip gaps, i.e., Absolute magnification error=Σi DXi, where i=the total number of chips, the bow/skew error is equal to the sum of “DY” for all chip gaps, i.e., Bow/skew error=Σi DYi, where i=the total number of chips, and bow may also be defined as P−P×DY, i.e., the error after skew is removed.
To address the potential imaging uniformity problems caused by the foregoing non-idealities, most LPB suppliers strive to minimize chip gaps and total multi-chip bow to an acceptable level for the desired print quality. The achievable placement of LED chips is usually adequate for a single LPB or monochrome print engine. However, this may not be the case for high quality monochrome printers or color printers where color to color registration is critical. Some suppliers may output chip gap or bow information in some format to enable some level of correction. While this technique may allow bow correction, it does not allow skew correction necessary for color registration. In addition, if a LPB is used as an nth color in a printer with a scanning laser imager, the corrected bow of the LPB may not match the non-zero bow of the laser imager. Moreover, the foregoing uniformity problems may be amplified or altered during use of the LPB as thermal changes to the LPB cause further chip and/or LED displacement due to the expansion or contraction of materials.
For example, gaps between chips in the cross process direction change as the temperature of the LPB changes. The material used to form printed circuit board 68 typically has a greater coefficient of thermal expansion than the material used to form chips 64. Thus, changes in temperature cause greater changes in the distance between chips 64 than the distances between LEDs 66. It is believed that due to the construction of the LPB, the locations of the largest expansion error will depend on how the LPB is mechanically mounted. For example, if the LPB is secured or pinned at one end, the largest error will be located at the opposite end, and if the LPB is secured or pinned at its middle, the expansion moves from the center outward which creates the largest error at both ends of the LPB. Moreover, chips 64 are typically secured to circuit board 68 via an epoxy deposited on the rear surface of each chip 64 at approximately its center. The epoxy is non-rigid to permit some expansion and contraction of the epoxy as the chip and/or circuit board expands or contracts. In view of the foregoing, it should be appreciated that the spaces between chips 64, i.e., chip gaps, open or close with thermal changes to the circuit board and chips. All of the foregoing changes may occur uniformly or non-uniformly depending on whether the change of temperature of the chips and circuit board occurs uniformly or non-uniformly.
Apparatus and methods to deal with this potential imaging uniformity problem under constant temperature conditions have been proposed; however, additional imaging error can be induced by thermal expansion of the print bar if the ambient temperature fluctuates during printing, or if the total print duty cycle fluctuates and causes the temperature delta between the LPB and ambient to vary. The main way this problem has been dealt with in LPB imaging systems is the technological progress in getting a reduction of LED power needed for a given exposure due to improved LED efficiency, heat sinks, active cooling, or some combination of the foregoing. Other means have been considered to remove or minimize the effects of thermal expansion. For example, reduction of gap sizes, reduction of the expansion areas, etc. have been employed. However, none of these means have be sufficient to satisfy performance requirements.
The apparatus and method disclosed herein address these problems without incurring the cost of additional heat sinking and cooling.